High shrink polyethylene films

ABSTRACT

A homogeneous blend of a low density polyethylene with a metallocene-catalysed polyethylene having a density of from 0.906 g/cm 3  and a Dow Rheology Index of at least 5/MI 2 , MI 2  being the melt index measured according to ASTMD-1238 condition 190° C./2.16 kg and the Dow Rheology Index being determined by a dynamic rheological analysis performed at 190°.

The present invention relates to polyethylene compositions and highperformance shrink films thereof combining excellent mechanicalproperties such as stiffness and toughness with good processability andgood optical properties. These polyethylene compositions can thereforebe used for film applications requiring this unique combination ofproperties, such as but not exclusively, packaging.

Shrink film has been used for years in the packaging industry to wraparticles. The process comprises packing the article and submitting it toheating in an oven, whereby the film is retracted so as to render thepacking tight and suitable to its end use.

It is well known to use linear low density polyethylene (LLDPE) in blendwith low density polyethylene (LDPE) in shrink film compositions.Compositions comprising of from 20 to 40% by weight of LLDPE with 80 to60% by weight of LDPE are commonly used. Indeed the addition of LLDPE toLDPE in shrink film compositions is well known in order to avoid theformation of holes that could occur during the retraction of shrink filmmade from pure LDPE.

Nevertheless the currently available polyethylene resins suffer frommajor drawbacks.

The low density polyethylene (LDPE) resins exhibit excellent optical andprocessing properties but they have poor mechanical properties and poorrigidity.

Linear low density polyethylene (LLDPE) resins have excellent mechanicalproperties but have mediocre optical properties and poor processability.Indeed LLDPE leads to bubble instability and its extrusion is difficult.If mixed with LDPE they have improved processability properties buttheir mechanical properties are reduced.

Metallocene-catalysed linear low density polyethylene (mLLDPE) resinshave excellent mechanical properties but poor optical properties andprocessability requiring extrusion equipment specially designed formLLDPE with wide die gap. If mixed with LDPE they have very good opticaland good sealing properties, but the mechanical properties are reduced.

Wherever high rigidity is needed, LDPE and LLDPE compositions willrequire overly thick structures. Especially for LLDPE, where excellentimpact and tear properties render its down-gauging capability useful,the lack of rigidity is a main drawback because high rigidity is arequirement for product packaging.

WO 95/27005 discloses mixtures of LDPE with LLDPE or mLLDPE. Therigidity of the mixtures is insufficient.

EP-A-0844277 discloses metallocene-catalysed medium density polyethylenewith LDPE and/or LLDPE compositions for blown films that claim a goodbalance between the good optical properties of LDPE and the goodmechanical and processing properties of medium density polyethylene(MDPE). However this specification does not address the problem of theproduction of shrinkable polyethylene films

EP-A-1108749 relates to shrink films of blended LDPE and MDPE resins.However those resins can still be further improved.

It is an object of the present invention to provide polyethylenecompositions for mono or multilayers films, that achieve good balancedshrink properties in machine direction (MD) and transverse direction(TD) with fast shrink speed and high cohesion force at room temperaturewhile keeping a good rigidity, excellent optical properties and an easyprocessing in film blowing process.

In the present invention, a film is defined as an extremely thincontinuous sheet: the upper limit for thickness is of about 250 microns(Hawley's Condensed Chemical Dictionary, Twelfth Edition, Rev. by R. J.Lewis, Van Nostrand Reinhold Co., New York)

In the present invention, good balanced shrink properties in machine andtransverse directions of the polyethylene resin produced according tothe invention is defined as a resin having a shrinkage value intransverse direction of at least 5%, preferably of at least 10% comparedto known LDPE/LLDPE film while keeping shrinkage value in machinedirection similar to LDPE/LLDPE film, LDPE/LLDPE films and filmsaccording to the blend of the invention being extruded in the sameconditions.

In the present context, high cohesion force at room temperature of thepolyethylene resin produced according to the invention is defined as aresin having a cohesion force in transverse direction greater than 5%,preferably greater than 10% compared to LDPE/LLDPE films.

Fast shrink speed of the polyethylene resin produced according to theinvention is defined as a resin that shrinks of at least 10%, morepreferably at least 20% faster than LDPE/LLDPE films.

Good optical properties of the polyethylene film produced according tothe invention is here defined as a film having a gloss at an angle of45° of at least 60 and a haze of less than 10%.

This invention relates to a homogeneous blend of a low densitypolyethylene (LDPE) with a metallocene-catalysed polyethylene (mPE)having a density of from 0.906 g/cm³ and a Dow Rheology Index (DRI) ofat least 5/MI₂, MI₂ being the melt index measured according toASTMD-1238 condition 190° C./2.16 kg and the Dow Rheology Index beingdetermined by a dynamic rheological analysis performed at 190°, thisblend consisting of from 0.5% to 99.5% by weight of mPE and of from99.5% to 0.5% by weight of LDPE, based on the total weight of the blend.

In this specification, the density of the polyethylene is measured at23° C. using procedures of ASTM D-1505 and the melt index is measuredaccording to ASTM D-1238 condition 190° C./2.16 kg.

Other polymers compatible with said blend can be added to the blend to atotal amount not to exceed 33% by weight based on the total weight ofthe polymers.

The densities of the mPE used in the present invention are regulated bythe amount of comonomer injected in the reactor; they will range from0.906 g/cm³ to less than 0.965 g/cm³. Examples of comonomer which can beused include 1-olefins such as propylene, butene, hexene, octene,4-methyl-pentene, and the like, as well as mixtures thereof up to C121-olefins, the most preferred being hexene. Ethylene can also be used assuch without any addition of comonomers. Homopolymers of ethylene arethen produced.

According to one embodiment, the density of the mPE will range from0.906 g/cm³ to less than 0.925 g/cm³.

According to another embodiment, the density of the mPE will range from0.925 g/cm³ to less than 0.965 g/cm³ preferably from 0.925 g/cm³ to lessthan 0.950 g/cm³.

The melt index of the mPE used in the present invention can be regulatedby the amount of hydrogen injected in the reactor; it will range from0.1 g/10′ to 15 g/10′, preferably from 0.2 g/10′ to 4 g/10′

The molecular weight distribution (D) defined as the ratio between theaverage molecular weight by weight (Mw) and the average molecular weightby number (Mn) of the mPE used in the present invention is of from 2 to8, preferably of from 2 to 4 and even more preferably of from 2 to 3.5.

The melt flow ratio of the mPE used in the present invention is of from25 to 100. The melt flow ratio being the ratio HLMI/MI₂, the HLMI beingmeasured according to ASTM D-1238, condition 190° C./21.6 kg and the MI₂being measured according to ASTM D-1238, condition 190° C./2.16 kg.

The mPE resin used in the present invention has a high Dow RheologicalIndex (DRI). To characterize the rheological behavior of substantiallylinear ethylene polymers, S Lai and G. W. Knight introduced a newrheological measurement, the Dow Rheology Index (DRI) which expresses apolymer's “normalized relaxation time as the result of long chainbranching” (ANTEC '93 Proceedings, Insite™ Technology Polyolefins(ITP)—New Rules in the Structure/Rheology Relationship of Ethylene&-Olefin Copolymers, New Orleans, La., May 1993). S. Lai et al definedthe DRI as the extent that the rheology of ethylene-octene copolymersknown as ITP (Dow's Insite Technology Polyolefins) incorporating longchain branching into the polymer backbone deviates from the rheology ofthe conventional linear homogeneous polyolefins that are reported tohave no long chain branching by the following normalized equation:DRI=(365000(t ₀/η₀)−1)/10wherein t₀ is the characteristic relaxation time of the material and η₀is the zero shear viscosity of the material (Antec '94, Dow RheologyIndex (DRI) for Insite™ Technology Polyolefins (ITP): Uniquestructure-Processing Relationships, pp. 1814-1815). The DRI iscalculated from the best fitting by least squares analysis of therheological curve (complex viscosity versus frequency) as described inU.S. Pat. No. 6,114,486 with the following generalized Cross equation,i.e.η=η₀/(1+(γt ₀)^(n))wherein n is the power law index of the material, η and γ are themeasured viscosity and shear rate data respectively. The dynamicrheological analysis is performed at 190° C. and the strain amplitude is10%. Results are reported according to ASTM D 4440.

The DRI of the mPE used in the present invention is at least 5/MI₂,preferably at least 10/MI₂, more preferably at least 20/MI₂.

It has been observed that when the dynamic rheological analysis isperformed at a temperature lower than 190° C., higher DRI values can beobtained compared to those obtained when the dynamic rheologicalanalysis is performed at a temperature of 190° C. and vice versa.

Attention should thus be paid to the temperature at which the dynamicrheological analysis is performed when DRI values are compared.

DRI values ranging from zero for polymers which do not have measurablelong chain branching to about 15 are known and have been described inseveral U.S. patents such as for example U.S. Pat. Nos. 6,114,486,5,674,342, 5,631,069.

The manufacture of the low density polyethylenes used in the presentinvention is known in the art and is described for example in“Encyclopedia of Polymer Science and Engineering”, second edition,Volume 6, on pages 404 to 410 (LDPE) and pages 436 to 444 (LLDPE).

The catalyst system used to produce the polyethylene required by thepresent invention comprises a metallocene component. The metallocenecomponent can be any metallocene component known in the art of thegeneral formulas:(Cp)_(m)MR_(n)X_(q)   I.wherein Cp is a cyclopentadienyl ring, M is a group 4b, 5b or 6btransition metal, R is a hydrocarbyl group or hydrocarboxy having from 1to 20 carbon atoms, X is a halogen, and m=1-3, n=0-3, q=0-3 and the summ+n+q is equal to the oxidation state of the metal.(C₅R′_(k))_(g)R″_(s)(C₅R′_(k))MQ_(3−g)   II.andR″_(s)(C₅R′_(k))₂MQ′  III.wherein (C₅R′_(k)) is a cyclopentadienyl or substitutedcyclopentadienyl, each R′ is the same or different and is hydrogen or ahydrocarbyl radical such as alkyl, alkenyl, aryl, alkylaryl, orarylalkyl radical containing from 1 to 20 carbon atoms or two carbonatoms are joined together to form a C₄-C₆ ring, R″ is a C₁-C₄ alkyleneradical, a dialkyl germanium or silicon or siloxane, or a alkylphosphine or amine radical bridging two (C₅R′_(k)) rings, Q is ahydrocarbyl radical such as aryl, alkyl, alkenyl, alkylaryl, or arylalkyl radical having from 1-20 carbon atoms, hydrocarboxy radical having1-20 carbon atoms or halogen and can be the same or different from eachother, Q′ is an alkylidene radical having from 1 to about 20 carbonatoms, s is 0 or 1, g is 0, 1 or 2, s is 0 when g is 0, k is 4 when s is1 and k is 5 when s is 0, and M is as defined above.

Among the preferred metallocenes used one can cite among others bistetrahydro-indenyl compounds and bis indenyl compounds as disclosed forexample in WO 96/35729. The most preferred metallocene catalyst isethylene bis (4,5,6,7-tetrahydro-1-indenyl) zirconium dichloride.

The metallocene may be supported according to any method known in theart. In the event it is supported, the support used in the presentinvention can be any organic or inorganic solids, particularly poroussupports such as talc, inorganic oxides, and resinous support materialsuch as polyolefin. Preferably, the support material is an inorganicoxide in its finely divided form and has a surface area comprisedbetween 100 and 1200 m²/g.

An active side must be created by adding a cocatalyst having an ionizingaction. While alumoxane can be used as cocatalyst, it is not necessaryto use alumoxane as cocatalyst during the polymerization procedure forpreparing the mPE resin. When alumoxane is used as a cocatalyst, anyalumoxane known in the art can be used. The preferred alumoxanescomprise oligomeric linear and/or cyclic alkyl alumoxanes represented bythe formula:

for oligomeric, linear alumoxanes and

for oligomeric, cyclic alumoxanes,wherein n is 1-40, preferably 10-20, m is 3-40, preferably 3-20 and R isa C1-C8 alkyl group and preferably methyl.

Methylalumoxane is preferably used.

The amount of alumoxane and metallocene usefully employed in thepreparation of the solid support catalyst can vary over a wide range.Preferably the aluminium to transition metal mole ratio is in the rangebetween 20:1 and 500:1, preferably in the range 50:1 and 300:1.

When alumoxane is not used as a cocatalyst, one or more aluminium alkylrepresented by the formula AlR_(x) are used wherein each R is the sameor different and is selected from halides or from alkoxy or alkyl groupshaving from 1 to 12 carbon atoms and x is from 1 to 3. Especiallysuitable aluminiumalkyl are trialkylaluminium, the most preferred beingtriisobutylaluminium (TIBAL).

In the present invention, the mPE can be monomodal, bimodal ormultimodal.

The metallocene catalyst utilized to produce the polyethylene requiredby the present invention can be used in gas, solution or slurrypolymerizations. Preferably the polymerization process is conductedunder slurry phase polymerization conditions. It is preferred that theslurry phase polymerization conditions comprise a temperature of from 20to 125° C., preferably from 60 to 110° C. and a pressure of from 0.1 to8 MPa, preferably from 2 to 5 MPa for a time between 10 minutes and 4hours, preferably between 0.4 and 2.5 hours. High pressure range like100 to 2000 bars can be used for polymerization in high pressure tubularor autoclave reactors.

It is preferred that the polymerization reaction be run in a diluent ata temperature at which the polymer remains as a suspended solid in thediluent. Diluents include, for examples, propane, isobutane, n-hexane,n-heptane, methylcyclohexane, n-pentane, n-butane, n-decane, cyclohexaneand the like as well as mixtures thereof. The preferred diluent isisobutane. The diluent can be under liquid or super critical state.

The polymerization of the mPE used in the present invention can beconducted in a continuous reactor. The continuous reactor is preferablya loop reactor. During the polymerization process, at least one monomer,the catalytic system and a diluent are flowed in admixture through thereactor.

Alternatively for a bimodal production of mPE, two reactors in seriescan be used.

In the present invention average molecular weights can be furthercontrolled by the introduction of some amount of hydrogen or by changingthe temperature during polymerization. When hydrogen is used it ispreferred that the relative amounts of hydrogen and olefin introducedinto the polymerization reactor be within the range of about 0.001 to 15mole percent hydrogen and 99.999 to 85 mole percent olefin based ontotal hydrogen and olefin present, preferably about 0.02 to 3 molepercent hydrogen and 99.98 to 97 mole percent olefin.

Standard additives such as antioxidants may be used for both long termand processing stabilization and if desired, one or more pigments and/ordyes and/or processing aids like fluoro elastomers can also be added.

Antistatic, antifog, antiblocking or slip additives may also be added.

According to embodiments of the present invention, compositions of LDPEand mPE are obtained either by preliminary dry blend or extrusion or bydirect blend in the hopper or via the extruder.

The present invention further provides the use of the homogeneous blendaccording to the present invention to produce a monolayer blown film orone or more layers of a multilayer blown film wherein said layers arearranged in any order.

The present invention still further provides films, prepared with theblend of the present invention, characterised by:

-   -   a shrinkage value in transverse direction of more than at least        5%, preferably of more than at least 10% compared to LLDPE/LDPE        films extruded in the same conditions as those us for the blend        of the present invention.    -   a cohesion force in transverse direction at room temperature        greater than of at least 5%, preferably greater than 10%        compared to LDPE/LLDPE films.    -   a shrink speed value of at least 10% more preferably at least        20% faster than LDPE/LLDPE films.    -   A gloss at an angle of 45° of at least 60 and a haze of less        than 10%. while keeping a good rigidity and being easily        processable.

In the present invention, a good rigidity means e.g. that for a mPE/LDPEblend according to the invention, a secant modulus at 1% of deformationaccording to ASTM D-882 is:

-   -   at least of 200 mega Pascal for a mPE/LDPE blend having a        density of 0.920 g/cm³.    -   at least of 230 mega Pascal for a mPE/LDPE blend having a        density of 0.923 g/cm³.    -   at least of 260 mega Pascal for a mPE/LDPE blend having a        density of 0.926 g/cm³.    -   at least of 320 mega Pascal for a mPE/LDPE blend having a        density of 0.930 g/cm³,        the secant modulus at 1% of deformation according to ASTM D-882        for the pure LDPE being of 200 mega Pascal.

As a consequence of a higher shrinkage in transverse direction and ahigher cohesion force of the film produced according to the inventionand caused by the high DRI mPE, it will be possible to make thinnershrink film leading to a significant cost reduction.

Increasing the shrink speed of the film produced according to theinvention is particularly interesting because it results either inhigher shrink-wrapping packaging rates or in decreasing the temperatureof the oven during the packaging wrapping process, both results leadingto a significant cost reduction.

The blend as described here above may also be used in the production oflamination films, barrier films and reticulation applications.

These films may also be metallized, corona treated, printed andlaminated.

EXAMPLES

1. Polymerization Procedure and Product Composition.

The polymerization of the mPE (R1) used in the blend of the presentinvention was carried out in a liquid-full slurry loop reactor. Ethylenewas injected with 1-hexene together with the catalyst. Isobutane wasused as diluent. The polymerization conditions are indicated in Table ITABLE I Resin R1 C2 feed (kg/h) 3900 C6 feed (g/kg C2) 22 H2 feed (g/t)42 Iso C4 feed (kg/h) 1940 Tibal conc (ppm) 100-200 T. pol (° C.) 90C2 = ethyleneC6 = 1-hexeneIso C4 = isobutaneTibal = triisobutylaluminium

The bridged metallocene catalyst used was ethylene bis(tetrahydro-indenyl) zirconium dichloride

R1 produced according to the above polymerization conditions is a highDRI mPE.

The data concerning R1 in comparison with a purely linear metallocenemedium density from Phillips identified as Marlex mPact D350® aresummarised in Table II. TABLE II R1 Marlex mPact D350 ® Density g/cm³0.934 0.933 MI₂ g/10 min 0.9 0.9 DRI 36 0 SR2 = HLMI/MI₂ 30 16 D (Mw/Mn)3.28 2.88

The DRI of R1 and Marlex mPact D350® were determined by fitting thegeneralized Cross equation on the complexe viscosity measured accordingto ASTM D4440 by using a RDA 700 from Rheometrics, a diameterplate-plate of 25 mm and a gap between plates of 2 mm +/−0.2 mm. Theapparatus was callibrated according to ARES Instrument normal 902-30004The rheological measurements were performed at 190° C. under nitrogenand at 10% of strain.

2. Blends Preparation.

A blend B1 according to the invention was prepared by mixing 30% byweight of mPE (R1) and 70% by weight of LDPE

A blend B2 according to the invention was prepared with the sameingredients in different proportions: 70% by weight of R1 and 30% byweight of LDPE.

A comparison blend B3 was prepared by mixing 30% by weight of LLDPE and70% by weight of LDPE

A comparison blend B4 was prepared by mixing 30% by weight of the MarlexmPact D350® from Phillips and 70% by weight of LDPE.

The LDPE used in all the blends is characterised by a density of 0.924g/cm³ and a MI₂ of 0.8 g/10 min

The LLDPE used in comparison blend B3 is a Ziegler-Natta LLDPEcharacterised by a density of 0.918 g/cm³, a DRI of 0.6 and a molecularweight distribution (Mw/Mn) of 4.1.

3. Films Preparation.

Six films were blown on a Macchi blown film line equipment using a lowdensity configuration characterised by a die of 120 mm, a blow up ratioof 2.5:1 and a die gap of 0.8 mm. All the films were down-gauged to athickness of 40 microns.

Films F1 and F2 were prepared from the blend B1 and B2 according to theinvention.

Films F3 and F4 were prepared from the comparative blends B3 and B4.

Film F5 was prepared from pure LDPE

Film F6 was prepared from pure resin R1

4. Films Properties.

The shrinkage in machine direction (MD) and transverse direction (TD)was measured by immersion of the film in a bath of hot oil at 140° C.during 2 minutes according ASTM D2732.

The percentage of shrinkage for each direction (MD and TD) of the sixfilms are given in table III TABLE III Shrinkage Shrinkage TD FilmsComponent % weight MD (%) (%) F1 R1/LDPE 30/70 71 39 F2 R1/LDPE 70/30 6937 F3 LLDPE/LDPE 30/70 70 36 F4 mPact D350 ®/LDPE 30/70 70 33 F5 LDPE100 70 42 F6 R1 100 63 25

Pure LLDPE and Marlex mPact D350® could not be extruded by using anarrow die gap of 0.8 mm as desribed here above for the films F1 to F6.This is why no shrink results are given for those pure resins. Incontrast pure high DRI mPE (R1) could be extruded under thoseconditions. Moreover even pure it exibits good shrinkage in TDdirection.

It can also be observed from table III that the addition of high DRI mPEto LDPE increases the shrinkage of the film (F1) in transverse directioncompared to films (F3 and F4) produced from LLDPE/LDPE and mPactD350®/LDPE blends.

Table III shows clearly that the good shrink property in transversedirection is brought by the LDPE (film F5). For the shrink in machinedirection, all the blends studied (F1 to F4) exhibited the samebehaviour.

It is surprising that when the high DRI mPE is used even in highconcentration with LDPE the film produced (F2) still exibits bettershrinkage in transverse direction compared to the films using much moreLDPE (F3 and F4).

The cohesion force exerted by the film after shrink and cooling the filmat room temperature was measured according to ISO 14616 method bysetting the films in an oven at 180° C. during 17 seconds. The cohesionforce was then measured at room temperature for each film in the machinedirection (MD) and transverse direction (TD). Those forces are expressedin Newton (N).

The measurement results are shown in table IV. TABLE IV % MD Cohesion TDCohesion Films Components weight F. (N) F. (N) F1 R1/LDPE 30/70 1.1020.835 F2 R1/LDPE 70/30 1.21 1.01 F3 LLDPE/LDPE 30/70 0.902 0.622 F4mPact D350 ®/LDPE 30/70 1.1 0.62 F5 LDPE 100 0.85 0.722 F6 R1 100 1.354not applicable

When added to LDPE it is seen that LLDPE and mPact D350® increase thecohesion force of the film in machine direction (MD) but decrease it intransverse direction (TD) compared to pure LDPE (F3 and F4 versus F5).Contrary the high DRI mPE/LDPE blend not only increases substantiallythe cohesion force of the film in machine direction but also increasesthe cohesion force in transverse direction (TD) when compared to pureLDPE (F1 versus F5). Both cohesion forces are further increased whenincreasing amount of high DRI mPE with LDPE (F2 versus F1).

The cohesion force of the film issued from the LLDPE/LDPE blend ispartly related to the molecular weight distribution (D) defined as theratio between the average molecular weight by weight (Mw) and theaverage molecular weight by number (Mn) of the LLDPE. Higher is themolecular weight distribution of the LLDPE, better will be the cohesionforce of the film. It is remarkable that the blend according to theinvention while using a mPE having a narrower molecular weightdistribution compared to the LLDPE (3.28 versus 4.1) allows to get filmswith a better cohesion force than that produced from LLDPE/LDPE blend(F1 and F 2 versus F3).

It has also been unexpectetly found that the shrink speed in the machinedirection of the film produced according to the blend of the inventionis higher compared to those of the films produced from the LDPE/LLDPEand from the mPact D350® blends. Table V illustrates this result. TABLEV MD MD Elmendorff Secant shrink Tear Gloss Haze modulus Films time (s)(N/mm) 45° (%) (mPa) F2 (example) 15 33.5 60.7 7.3 280 F3 (comparative)20 25 57.4 8.3 189 F4 (comparative) 20 — 57.4 8.3 189

Besides a shorter shrink time, a higher MD Elmendorf tear resistance, abetter rigidity, a better gloss at 45° C. and less haze are alsoobserved for the film produced according to the invention.

It is unexpected and remarkable that it is the film according to theinvention that give higher tear resistance and better optical propertieseven if the density of the blend produced according to the invention ishigher than that of the comparative blend

The Elmendorf tear was measured using the method of ASTM D-1922.

The gloss was measured at an angle of 45° with the Byk-Gardnermicro-gloss reflectometer according to ASTM D2457, the haze was measuredwith the Byk-Gardner Hazegard® system according to ASTM D-1003.

The MD shrink time was measured using the method ISO 14616.

The Secant modulus at 1% of deformation was measured according to ASTMD-882.

1-8. (Cancelled)
 9. A polyethylene composition comprising a homogenousblend of a low density polyethylene with a metallocene-catalysedpolyethylene having a density of at least 0.906 g/cm³ and a Dow RheologyIndex of at least 5/MI₂, wherein MI₂ is the melt index measuredaccording to ASTMD-1238 condition 190° C./2.16 kg and the Dow RheologyIndex being determined by a dynamic rheological analysis performed at190°, said metallocene-catalysed polyethylene being catalysed with acatalyst system comprising a metallocene component and an activatingagent selected from the group consisting of an alumoxane or an aluminumalkyl providing a mole ratio of aluminum to the transition metal of saidmetallocene component within the range of 50:1-300:1.
 10. Thecomposition of claim 9 wherein the metallocene-catalysed polyethylenehas a Dow Rheology Index of at least 10/MI₂.
 11. The composition ofclaim 9 wherein the metallocene-catalysed polyethylene has a DowRheology Index of at least 20/MI₂.
 12. The composition of claim 9wherein the metallocene-catalysed polyethylene has a density of from0.925 g/cm³ to less than 0.965 g/cm³.
 13. The composition of claim 12wherein the metallocene-catalysed polyethylene has a Dow Rheology Indexof at least 10/MI₂
 14. The composition of claim 12 wherein themetallocene-catalysed polyethylene has a Dow Rheology Index of at least20/MI₂.
 15. The composition of claim 9 wherein the metallocene-catalysedpolyethylene has a density of from 0.906 g/cm³ to less than 0.925 g/cm³.16. The composition of claim 15 wherein the metallocene-catalysedpolyethylene has a Dow Rheology Index of at least 10/MI₂.
 17. Thecomposition of claim 15 wherein the metallocene-catalysed polyethylenehas a Dow Rheology Index of at least 20/M₂.
 18. A blown film comprisingat least one layer formed of a polyethylene composition comprising ahomogenous blend of a low density polyethylene with ametallocene-catalysed polyethylene having a density of at least 0.906g/cm³ and a Dow Rheology Index of at least 5/MI₂, wherein MI₂ is themelt index measured according to ASTMD-1238 condition 190° C./2.16 kgand the Dow Rheology Index being determined by a dynamic rheologicalanalysis performed at 190°, said metallocene-catalysed polyethylenebeing catalysed with a catalyst system comprising a metallocenecomponent and an activating agent selected from the group consisting ofan alumoxane or an aluminum alkyl providing a mole ratio of aluminum tothe transition metal of said metallocene component within the range of50:1-300:1.
 19. The film of claim 18 wherein said layer is a monolayerblown film.
 20. The film of claim 18 wherein said layer comprises atleast one layer of a multilayer blown film.
 21. A process for thepreparation of a blown film, forming said film in a blown film line froma polyethylene composition comprising a homogenous blend of a lowdensity polyethylene with a metallocene-catalysed polyethylene having adensity of at least 0.906 g/cm³ and a Dow Rheology Index of at least5/MI₂, wherein MI₂ is the melt index measured according to ASTMD-1238condition 190° C./2.16 kg and the Dow Rheology Index being determined bya dynamic rheological analysis performed at 190°, saidmetallocene-catalysed polyethylene being catalysed with a catalystsystem comprising a metallocene component and an activating agentselected from the group consisting of an alumoxane or an aluminum alkylproviding a mole ratio of aluminum to the transition metal of saidmetallocene component within the range of 50:1-300:1, said film beingcharacterized by: a cohesion force in the transverse direction at roomtemperature of at least 5% greater than the cohesion force in thetransverse direction of a corresponding biaxially-oriented film formedof said low density polyethylene in a pure form; and a gloss at an angleof 45° of at least 60 and a haze of less than 10% while keeping a goodrigidity.
 22. A biaxially-oriented film which has been oriented in themachine direction and the transverse direction, formed from a homogenousblend of a polyethylene composition comprising a homogenous blend of alow density polyethylene with a metallocene-catalysed polyethylenehaving a density of at least 0.906 g/cm³ and a Dow Rheology Index of atleast 5/MI₂, wherein MI₂ is the melt index measured according toASTMD-1238 condition 190° C./2.16 kg and the Dow Rheology Index beingdetermined by a dynamic rheological analysis performed at 190°, saidmetallocene-catalysed polyethylene being catalysed with a catalystsystem comprising a metallocene component and an activating agentselected from the group consisting of an alumoxane or an aluminum alkylproviding a mole ratio of aluminum to the transition metal of saidmetallocene component within the range of 50:1-300:1, said film having acohesion force in the transverse direction which is greater than thecohesion force in the transverse direction of a correspondingbiaxially-oriented film formed of said low density polyethylene in apure form.
 23. The biaxially-oriented film of claim 22, wherein saidfilm has a cohesion force in the machine direction which is greater thanthe cohesion force in the machine direction of the corresponding filmformed of said low density polyethylene in a pure form.
 24. The film ofclaim 23, having a gloss at an angle of 45° of at least 60, and a hazewhich is less than 10%.